As electronic appliances have been rendered small and portable in recent years, there has been increased a demand for lithium secondary batteries having a light weight and a high energy density. Among them, a spinel type of LiMn.sub.2 O.sub.4 which may be utilized as a positive electrode active material for 4 V grade of lithium ion secondary batteries has been drawn attention for the use as the battery material for general use product such as automobile and the like since the raw material for manganese is abundant in resource and inexpensive.
Although spinel type of LiMn.sub.2 O.sub.4 is a material having relatively high battery capacity, it has a problem in cycle characteristics during charge and discharge, especially cycle characteristics under an elevated temperature (50.degree. C. or more) or in the elution of Mn into an electrolyte.
In order to solve such a problem, there has been proposed substitution of a part of Mn present in the spinel LiMn.sub.2 O.sub.4 structure for other metal. MnO.sub.6 octahedron wherein oxygen atoms are arranged in 6 directions is structually unstable because Mn located at the center of the structure has a large ionic radius so that clearance between oxygen atoms which are located at the outside of the octahedron becomes much. On the contrary, by substituting a part of Mn ion located at the center of the octahedron for an ion which is smaller than Mn, the structure is stabilized for the reason that the clearance between oxygen atoms becomes little so that a good packing is realized. Furthermore, it is expected that the reactivity of the other metal substituted product toward electrolyte is lowered so that the elution of Mn into the electrolyte becomes remarkably little.
As a process for preparing Mn complex oxides wherein a part of Mn is substituted for another metal, there is disclosed Al-substitution process (Japanese Laid-Open Patent Application Nos. Hei 4-289662, Hei 2-220358 etc.), B-substitution process (Japanese Laid-Open Patent Application No. Hei 8-195200), Fe- and Cr-substitution process (Japanese Laid-Open Patent Application No. Hei 9-245836) or the like. However, since each of these prior art processes is the so-called dry process wherein a water-insoluble Mn oxide or hydroxide and other metal component are mixed in a powdered form and subsequently heated, there has not yet been obtained complex oxide wherein metal to be substituted is uniformly doped with Mn. Therefore, there has not yet been obtained complex oxide which had eliminated sufficiently such fault of LiMn.sub.2 O.sub.4 as in the charging and discharging cycle characteristics, especially cycle characteristics at an elevated temperature (50.degree. C. or more) or in the elution of Mn into an electrolyte.
Also, although Mn series of secondary batteries have been drawn attention as the electric source material for use in an electric automobile or notebook type of personal computer, they are often used under an environment of relatively higher temperature than the ordinary temperature, and therefore, the improvement in the charging and discharging cycle characteristics at an elevated temperature has been desired.